In recent years, a transmission system that performs optical transmission of large volumes of data by using an optical network is desired. In a transmission system for optical networks, for example, a multicarrier modulation technique, such as a discrete multi-tone (DMT) modulation technique or the like, is known. The DMT modulation technique is one of the multi carrier transmission technologies based on the orthogonal frequency division multiplexing (OFDM) technology. The DMT modulation technique is a technology that allocates data to subcarriers (SCs) with a plurality of different frequencies, that modulates the data allocated to each SC on the basis of the modulation level and the allocation amount of signal power, and that transmits the data as DMT signals at a high speed.
A transmission device used in The DMT modulation technique transmits, at the time of a startup of a system or the like, a probe signal to a transmission device on an opposite side. Then, the transmission device on the opposite side measures the transmission characteristic on the basis of the received probe signal and sends the subject transmission characteristic as a reply to the transmission device. Then, the transmission device determines, on the basis of the received transmission characteristic, the modulation level (number of bits) and the allocation amount of signal power for each SC. Then, the transmission device generates a DMT signal by modulating the data allocated to each SC on the basis of the determined modulation level and the allocation amount of signal power for each SC. Namely, by performing a negotiation between transmission devices, stable DMT transmission can be ensured between the transmission devices.
Patent Document 1: Japanese Patent No. 5708252
Non-Patent Document 1: D. Lewis, et al. “400GE DMT Multi-Vendor Interoperability Requirements” http://www.ieee802.org/3/bs/public/15_01/corbeil_3bs_01_0115.pdf
If transmission devices are connected by a Point to Point network, because communication is two-way direction communication, the transmission characteristic based on a probe signal between transmission devices can be sent back. Thus, negotiations can be performed with the transmission device on an opposite side. Consequently, stable DMT transmission can be ensured on the basis of the transmission characteristic between the transmission devices.
However, if transmission devices are connected by a ring network, because communication is one-way direction communication, the transmission characteristic obtained on the basis of a probe signal between transmission devices is not able to be sent back. Thus, it is not able to ensure negotiations with the transmission device on an opposite side.